Detailed information must be acquired of the properties and behavior of the proteolytic enzyme, intestinal enterokinase, before a detailed description can be given of how digestive enzymes are activated and regulated. The molecular and enzymatic properties of bovine enterokinase will be compared with the serine proteinases, particularly the pancreatic proteases and the blood-clotting protein components. Partial amino acid sequence analysis and chemical modification of surface amino acid residues of enterokinase will be compared with the known sequence and conformation of other serine proteases. Amino acid residues critical for the biological function of the enzyme will be identified from chemical reactivities and enzymic activities. These studies will help to clarify the metabolic role of enterokinase as the initiator of the activation process producing active proteolytic enzymes in mammalian digestion.